The invention relates to a method and a device for producing a homogeneous sheet of quartz glass without streaks.
The absence of bubbles, the absence of streaks and homogeneity with respect to the refractive index are important when producing quartz glass plates. In this connection, a method and a device were disclosed in the DE 32 26 451 C2, for which the free end of a freely suspended solid quartz glass cylinder protrudes into a vessel, in an atmosphere of inert gas, and into a graphite melting pot in the vessel. The solid quartz glass cylinder is heated in the vessel to its flow temperature. After it reaches its flow temperature, it is lowered to the bottom of the graphite melting pot and advanced in order to melt it. Aside from the fact that considerable technological effort is required for this purpose, the suspension is less stable and the yield of quartz glass with small changes in refractive index is very limited. Moreover, the remainder of the solid quartz glass cylinder can be removed only after the device has cooled down. This is associated with great expense and glass loss.
It is therefore an object of the invention to provide a method and a device, which function technologically with little effort and make improved quality characteristics possible, especially with respect to the homogeneity of the refractive index over a large portion of the quartz glass plate.
Pursuant to the invention, this objective is accomplished by providing a quartz glass body which is cylindrical preferably and optically free of streaks at least in the longitudinal direction, as a blank with a relatively small cross section. The blank is transformed into a sheet of almost any configuration and extent, which has a relatively large homogeneous region and is determined only by the geometry of a melting pot, consisting advantageously of graphite, which is a graphite mold. At the same time, the relative movement between the quartz glass object and the melting pot, which is directed transversely to the longitudinal axis of the quartz glass rod, preferably is a rotational movement about an axis directed essentially parallel to the above-mentioned longitudinal axis. The relative movement in the present invention can thus be composed of a single transverse shifting and a constant rotational movement. For carrying out the relative movement between the quartz glass rod and the melting pot, only the quartz glass rod or the melting pot advantageously is moved. Advantageously, the quartz glass rod is introduced eccentrically into the melting pot, so that its longitudinal axis does not pass through the center of the melting pot or, in the event that the melting pot is cylindrical in shape, is offset essentially parallel to the cylindrical axis of the melting pot. The eccentricity then advantageously is equal to the radius of the quartz glass rod, which is also constructed as a cylinder. For melting the quartz glass rod, the latter is not lowered to the bottom of the melting pot, but kept at a certain distance from the bottom, so that there is nothing to impede the melting heat reaching a free end of the quartz glass rod in the melting pot from all sides. In the course of the melting, the quartz glass rod is advanced at a suitable and optionally adjustable rate. When the quartz glass plate attains the desired thickness, the advance of the quartz glass rod is suspended. The flow temperature is still retained for a certain time, the quartz glass rod is moved away from the quartz plate, and at the same time, melted off from its holding device. Heating is subsequently suspended. Advantageously, the melting-off site is flushed with inert gas. The whole process can take place automatically.
For the manufacturing process, it is an advantage if the quartz glass rod and the melting pot are constructed cylindrically. The quartz glass rod is connected to an arm over a constriction and an interposed holding piece. The melting off at the end of the melting process for producing the plate advantageously takes place at the constriction. For adjusting the eccentricity, the arm advantageously is constructed telescopically. In order to carry out the relative movement of the quartz glass rod relative to the melting pot, either the quartz glass rod can be mounted on the arm or preferably the melting pot can be mounted in a furnace housing or both can be mounted rotatably. If the melting pot is in the furnace housing, then the latter advantageously is provided at its inside with heating elements in a suitable arrangement. For introducing the quartz glass rod into the melting pot, the furnace housing and the heating space are provided with lead-throughs, of which at least the lead-through in the furnace housing is equipped so that it can be flushed with an inert gas.
The method of the present invention is characterized by the following steps. The preferably cylindrical quartz glass column or quartz glass rod is transformed thermally in a graphite mold heated in a heatable furnace housing. For this purpose, the lower end of the quartz glass rod, guided by means of a holding device, is lowered up to a certain distance above the bottom of the graphite mold in the furnace. This distance can be 5 to 20 mm and serves for heating uniformly as well as for ensuring a certain safety. By being heated to its flow temperature, the quartz glass rod melts off under the action of its own weight and flows onto the bottom of the mold. While continuing the heating, the quartz glass rod is lowered further at a defined rate up to a pre-selected height mark, which fixes the lower edge of a melting-off position. After this height mark is reached, the remaining quartz glass rod is held in this position for a certain time, before it is pulled out of the furnace at a defined rate while simultaneously melting off. At the same time, no melt is pulled out of the melting pot. After the melting off commences, there is a relative rotary and/or translatory movement between the quartz glass rod and the graphite mold at right angles to the direction, in which the quartz glass rod is lowered, preferably at a rate of 0.1 to 3 revolutions per minute. At the same time, the geometric axes of the quartz glass rod and the graphite mold can preferably be offset mutually by the radius of the quartz glass rod. By means of this measure, which also ensures that the interior glass and the surface glass of the quartz glass rod are not mixed with one another, the quartz glass plate, being formed in the graphite melting pot, has more advantageous homogeneity relationships than does the starting glass rod. That is, the cross-sectional region of defined good quality is relatively larger in the molten quartz glass plate that in the total cross section, relative to the total cross section of the quartz glass rod. Taking into consideration a height marking, a melting-off site has been incorporated in the quartz glass rod in the form of a constriction or a welded-in intermediate piece, up to which the quartz glass of the rod is used for the production of the quartz glass plate in the graphite melting pot. A further lowering of the quartz glass rod then does not take place. It is advantageous to flush not only the interior of the furnace with inert gas, but also the inlet opening to the furnace for the quartz glass rod and, with that, the quartz glass rod or the holding rod for the quartz glass rod at this site. The quartz glass rod consists of a material of high quality or of a quality adequate for the quartz glass plate, which is to be produced. The holding rod or the corresponding holding elements, preferably welded to the quartz glass rod, consist of materials of a quality, which can be lower that of the quartz glass rod.